The invention relates to the field of optical semiconductors, and in particular to the fabrication of optical semiconductor devices on a crystalline semiconductor layer and simultaneously on a reflective element by a simple, robust and high-yield production process.
Many optical devices are composed of a number of device layers, such as quantum wells, semiconductor junctions (e.g. pn-junctions), buffer layers, electron barriers etc., and at least one reflective element such as a backmirror. For example, many light emitting devices such as VCSELS consist of a bottom reflector, a semiconductor junction providing for light emission, and a top reflector. Other reflective optical devices consist of a reflector, on top of which a number of semiconductor layers are located that linearly or nonlinearly modify the reflected light. Examples of these elements are optical loss and phase modulators or semiconductor saturable absorbers.
One example includes optical devices having a substrate that provides for mechanical stability, heat dissipation and in some cases electrical conductivity, a reflective element underneath the device layers that reflects all or part of the incident light and can provide additional functionality such as heat and electrical conductivity, and finally the functional layers, herein referred to as device layers or epi-layers that provide the linear or nonlinear optical functionality of the device. In addition, on top of the device layers a top reflector can be deposited, which allows for the construction of Fabry-Perot type cavities such as in VCSELs.
All optical elements under discussion have in common that a crystalline substrate is necessary for deposition of the device layers, on which these layers can be grown. In addition, the surface, on which the device layers are deposited, should be of optical quality (i.e. low surface roughness, no scratches etc.) in order to avoid scattering losses in the device.
To date, most optoelectronic devices are made out of III-V semiconductors. One of the reasons for their success is that they can easily be integrated to provide for both reflective elements and device layers, and that the resulting elements can be produced in one processing step. On a semiconductor substrate such as GaAs or InP, layers out of III-V semiconductors can be grown lattice matched or slightly lattice mismatched, such that the crystalline structure, high surface quality and low defect density are preserved during the deposition process. Possible growth schemes are MBE, MOCVD or similar. In addition, a variation of the semiconductor compounds grown allows for variation of electrical and optical properties such as bandgap, Fermi level and refractive index. Thus, by deposition of a layer sequence with alternating high and low refractive index, a reflective element can be grown. Since the crystalline structure and high surface quality are preserved during the deposition process, the device layers can simply be deposited in the same growth process after deposition of the reflector.
However, one problem is that the refractive index between two layer materials that can be grown lattice matched is rather small. This problem is especially significant for devices based on InP, which can operate at the communications wavelength of 1.5 μm. A small index contrast makes it necessary to employ a large number of layers in the reflector to achieve a high reflectivity, and it leads to a narrow bandwidth and small acceptance angle of the reflector. This can lead to problems in applications, where a good electrical or thermal conductivity of the reflector is required, or where a wide bandwidth is necessary.
Numerous approaches to overcome these problems have been made in the past. First of all, III-V-based high reflectors based on material systems different from the standard GaAs and InP-based materials are under investigation. For example, a higher index material system could be InSb. Furthermore, for some of the applications discussed (semiconductor saturable absorbers and VCSELs) the reflector composed of III-V semiconductors has been replaced by dielectric or metallic reflectors. However, since the device layers must still be grown on a crystalline semiconductor film and since they cannot be grown on such a dielectric/metallic back-reflector, this requires a number of additional process steps. This adds complexity and reduces yield in the production process. One approach consists of first growing one or more etch stop layers lattice-matched on the substrate, then growing the device layers, then removing part of the substrate by etching from the back side and finally depositing of a dielectric/metallic back-reflector. However, besides the added complexity, this approach has a number of disadvantages.
The usable part of the wafer including device layers and back-reflector is small. In addition, the devices can have low mechanical stability and problems of heat dissipation since the devices are very thin, in the places where a back-reflector has been added. Finally, from the device side it is difficult to locate the functional device areas. Therefore, alternative approaches have been undertaken to add dielectric/metallic backmirrors to III-V semiconductor devices. For example, in one approach, first the device layer has been grown, then a dielectric/metallic reflector was deposited on top of it, and eventually the entire device was mounted with an adhesive on a new substrate such as quartz or copper.
These approaches are similar in that they include many steps in process technologies different from growth of the device layers, such that the devices can even have to be shipped to different production facilities for their completion. Besides the high complexity, the yield of all these processes is very low.
To overcome the problem of thick and narrowband backmirrors or of backreflectors that cannot be integrated with existing process technology, efforts have been made recently to develop semiconductor-based high-index contrast reflectors that can be integrated with existing process technology. These reflectors typically consist of two materials with a very large refractive index, for example silicon (Si) with an index n=3.5 and silicon dioxide (SiO2) with n=1.45. For simplicity this discussion will be limited to these two layer materials, but the invention is to be understood to be applicable to all possible material systems with a high index contrast. For example, the high index material could also be doped Si, Si-compounds such as Si(x)Ge(1−x), Ge etc and the low index material could be Si(x)O(y), SiN, ITO, and other heat or electrically conductive materials.
One method is to fabricate high-index contrast reflectors for use as back-reflectors in optical devices. According to this method, layer pairs of high- and low index are stacked on top of each other layer pair by layer pair by substrate bonding and smart-cut method to form the multilayer reflector. In this method all silicon layers are crystalline, and all surfaces can be high optical quality, such that the aforementioned problems are eliminated. However, this method does not allow for cost-effective batch processing (tens of wafers at a time). Furthermore, the repeated wafer bonding and cutting process leads to a low yield due to wafer breakage and bonding failures. Thus, this process produces the desired result at low yield and high cost.
Another method to fabricate high-index contrast reflectors is to deposit the multilayer stack comprising of low- and high index layers by vapor phase growth such as CVD. The benefit of this method is that it allows for cost-effective batch processing of a large number of wafers at a time, and that it avoids the yield-reducing wafer bonding technique. However, two problems result from this deposition method. First of all, the final surface is very rough, such that large scattering losses occur which can be detrimental to the device. It has been shown that the problem of scattering loss due to surface roughness can be overcome by flipping the multilayer stack, such that the layer deposited at the beginning of the growth appears topmost in the final device. The second problem of this method is that the semiconductor layers of the reflector are no longer crystalline, but amorphous or polycrystalline. As a result, they cannot be used as a substrate for subsequent growth of the device layers on top.